The atmosphere inside a bird hide is quite unusual, as interior spaces go. Low wind moaning in the drooping wires between telegraph poles. Whispering rushes and siffing seed heads of marsh grasses. Indistinguishable shifting murmurings, of the surrounding landscape, blown in through low letterbox windows. To the ear there is a lot of outside to be heard inside a bird hide. A fleeting curlew. A humming propeller plane. A distant pair of passing footsteps on the gravel towpath. 
But there are other sounds from inside to feel too. Interior sounds. Flurried sounds, made by internal things under external forces. Rattling shutters. Knocking slats. Timber panels grumbling. All set moving by wayward gusts of estuary air. And inbetween. When outside has less to say. Perfect, hidden, tranquility. 
As you sit quietly, on the wooden bench. And peer out through the narrow viewing slots to see what you can see, face brushed by fresh gusts of air, maybe just for a moment you realise what a bird hide is. A building trying not to be a building. A place trying not to be a place. A shelter that wants to hide you, but not be in your way. Spoil your view. Of the low tide water. The wide exposed mud flats. The silent birds, picking light footedly over the mud.